The invention relates to a push-pull amplifier comprising a first and a second output transistor of the same conductivity type, whose collector-emitter paths are arranged in series between two power-supply terminals, the base of the first transistor being coupled to an input terminal, the emitter of one transistor and the collector of the other transistor being coupled to an output terminal, which amplifier also comprises means for maintaining the sum of the base-emitter voltages of the first and the second transistor substantially constant.
Such a push-pull amplifier is known from U.S. Pat. No. 4,300,103. In order to ensure a satisfactory cross-over behavior in such a push-pull amplifier comprising output transistors of the same conductivity type, the sum of the base-emitter voltages of the output transistors is required to remain substantially constant. When the base-emitter voltage of the first transistor changes upon application of a drive voltage to this transistor, this should cause an equal but opposite change in base-emitter voltage of the second transistor. In the known circuit arrangement, this is achieved by measuring the a.c. signal across the base-emitter junction of the first transistor, forming a signal which is in phase opposition thereto, and driving the base-emitter junction of the second transistor with this opposite-phase signal.
However, such a push-pull amplifier has the drawback that the amplifier exhibits a substantial amount of second-harmonic distortion. This is because the conversion of the base-emitter voltage of the first transistor into the base-emitter voltage of the second transistors is relatively inaccurate, so that the first and the second transistor have different voltage gains, which gives rise to second-harmonic distortion.